Deep diode solid state transformer

ABSTRACT

An array of columnar structures are provided in a body of semiconductor material. The material of each columnar structure is recrystallized material of the body having solid solubility of dopant metal therein. Means are provided for connecting the columnar structures into two series electrical circuit arrangements to function respectively as the primary and secondary windings of a deep diode solid state transformer.

This is a continuation of application Ser. No. 410,999, filed Oct. 30,1973, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to solid state transformers and a process ofmaking the same.

2. Description of the Prior Art

Heretofore, solid state transformers have not been fabricated insubstrates of integrated circuits because of fabrication limitations.Circuit and chip designers employ a variety of circuits to design aroundthis process limitation.

An object of this invention is to provide a solid statr transformer forintegrated circuits.

Another object of this invention is to provide a process for making asolid state transformer.

Other objects of this invention will, in part, be obvious and will, inpart, appear hereinafter.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with the teachings of this invention, there is provided adeep diode solid state transformer. The transformer comprises a body ofsingle crystal semiconductor material having first and second majoropposed surfaces, a selected resistivity and a first type conductivity.A plurality of regions of second and opposite type conductivity and aselected resistivity is disposed in the body. Each region extendsbetween and terminates in the two major opposed surfaces and has twoopposed end surfaces. Each of the two end surfaces is coextensive withonly one of the major surfaces. The material of each of the regions isrecrystallized semiconductor material of the body having solidsolubility of a material therein to impart the second type conductivityand selective level of resistivity thereto. Each of the regions is a lowelectrical resistance path for conducting electrical currents betweenthe opposed surfaces of the body. A P-N junction is formed by thecontiguous surfaces of the materials of each region and the body. Firstmeans are provided for electrically connecting selective ones of theplurality of regions into a series circuit arrangement so as to functionas a primary winding of a transformer. Second means are provided forelectrically connecting the remaining ones of the plurality of regionsinto a series circuit arrangement so as to function as a secondarywinding of the same transformer. The regions are made by the thermalmigration of metal droplets embodying the temperature gradient zonemelting process.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top planar view of a body of semiconductor material beingprocessed in accordance with the teachings of this invention;

FIG. 2 is an elevation view, in cross-section of the body of FIG. 1taken along the cutting plane II--II;

FIGS. 3 and 4 are elevation views, in cross-section, of the body ofFIGS. 1 and 2 being processed further in accordance with the teachingsof this invention; and

FIG. 5 is an isometric view, partly in cross-section, of the deep diodesolid state transformer made in accordance with the teachings of thisinvention.

DESCRIPTION OF THE INVENTION

With reference to FIG. 1, there is shown a body 10 of semiconductormaterial having a selected resistivity and a first type conductivity.The body 10 has opposed major surface 12 and 14 which are the top andbottom surfaces respectively thereof. The semiconductor materialcomprising the body 10 may be silicon, germanium, silicon carbide,gallium arsenide, a compound of a Group II element and a Group VIelement, and a compound of a Group III element and a Group V element.

The body 10 is mechanically polished, chemically etched to remove anydamaged surfaces, rinsed in deionized water and dried in air. An acidresistant mask 16 is disposed on the surface 12 of the body 10.Preferably, the mask is of silicon oxide which is either thermally grownor vapor deposited on the surface 12 by any of the methods well known tothose skilled in the art. Employing well known photolithographicaltechniques, a photoresist, such, for example, as Kodak Metal EtchResist, is disposed on the surface of the silicon oxide layer 16. Theresist is dried by baking at a temperature of about 80° C. A suitablemask defining one or more geometrical shapes such, for example, as acircle or a square is disposed on the layer of photoresist and exposedto ultraviolet light. After exposure, the layer of photoresist is washedin xylene to open windows in the mask where the lines are desired so asto be able to selectively etch the silicon oxide layer 16 exposed in thewindows.

Selective etching of the layer 16 of silicon oxide is accomplished witha buffered hydrofluoric acid solution (NH₄ F--HF). The etching iscontinued until a second set of windows 17 corresponding to the windowsof the photoresist mask are opened in the layer 16 of silicon oxide toexpose selective portions of the surface 12 of the body 10 of silicon.The processed body 10 is rinsed in deionized water and dried. Theremainder of the photoresist mask is removed by immersion inconcentrated sulphuric acid at 180° C or immersion in a solution of 1part of hydrogen peroxide and 1 part of concentrated sulphuric acidimmediately after mixing.

Selective etching of the exposed surface of areas of body 10 isaccomplished with a mixed acid solution. The mixed acid solution is 10parts by volume nitric acid, 70%, 4 parts by volume acetic acid, 100%,and 1 part by volume hydrofluoric acid, 48%. At a temperature of from20° C to 30° C, the mixed acid solution selectively etches the siliconof the body 10 at a rate of approximately 5 microns per minute. Adepression 18 is etched in the surface 12 of the body 10 beneath eachwindow 17 of the oxide layer 16. The selective etching is continueduntil the depth of the depression 18 is approximately equal to thediameter or width of the window 17 in the silicon oxide layer 16.However, it has been discovered, that the depression 18 should not begreater than approximately 100 microns in depth because undercutting ofthe silicon oxide layer 16 will occur. Undercutting of the layer 16 ofsilicon oxide has a detrimental effect on the width of the device to bemigrated through the body 10. Etching for approximately 5 minutes at atemperature of 25° C will result in a depression 18 of from 25 to 30microns in depth for a window 17 of a diameter or width of from 10 to500 microns. The etched body 10 is rinsed in distilled water and blowndry. Preferably, a gas such, for example, as freon, argon and the like,is suitable for drying the processed body 10.

The processed body 10 is disposed in a metal evaporation chamber. Ametal layer 20 is deposited on the remaining portions of the layer 16 ofsilicon oxide and on the exposed silicon in the depressions 18. Themetal in the depressions 18 are the metal "wires" to be migrated throughthe body 10. The metal of the layer 20 comprises a material, eithersubstantially pure in itself or suitably doped by one or more materialsto impart a second and opposite type conductivity to the materials ofthe body 10 through which it migrates. The thickness of the layer 20 isapproximately equal to the depth of the depressions 18. Thus, the layer20 is approximately 20 microns in thickness. A suitable material for themetal layer 20 is aluminum to obtain P-type regions in N-type siliconsemiconductor material. Prior to migrating the metal wires in thetroughs 18 through the body of silicon 10, the excess metal of the layer20 is removed from the silicon oxide layer 16 by such suitable means asgrinding away the excess metal with a 600 grit carbide paper or byselective etching.

It has been discovered that the vapor deposition of the layer 20 ofaluminum metal should be performed at a pressure of approximately 1 ×10.sup.⁻⁵ torr but not less than 5 × 10.sup.⁻⁵ torr. When the pressureis greater than 3 × 10.sup.⁻⁵ torr, we have found that in the case ofaluminum metal deposited in the depression 18, the aluminum does notpenetrate into the silicon and migrate through the body 10. It isbelieved that the layer of aluminum is saturated with oxygen andprevents good wetting of the contiguous surfaces of silicon. The initialmelt of aluminum and silicon required for migration is not obtainedbecause of the inability of aluminum atoms to diffuse into the siliconinterface. In a like manner, aluminum deposited by sputtering is notdesirable as the aluminum appears to be saturated with oxygen from theprocess. The preferred methods of depositing aluminum on the siliconbody 10 are by the electron beam method and the like wherein little ifany oxygen can be trapped in the aluminum.

Referring now to FIG. 3, the processed body 10 is placed in a thermalmigration apparatus, not shown, and the metal in the depressions 18forms a droplet 22 of metal-rich alloy of the material of the body 10 ineach etched area of surface 12 and is migrated through the body 10 by athermal gradient zone melting process. A thermal gradient ofapproximately 50° C per centimeter between the bottom surface 14, whichis the hot face, and the surface 12, which is the cold face, has beendiscovered to be appropriate for an average temperature of the body 10of from 700° C to 1350° C. The process is practiced for a sufficientlength of time to migrate the metal-rich droplet 22 through the body 10.For example, for aluminum metal of 20 microns thickness, a thermalgradient of 50° C/centimeter, a temperature of the body 10 of 1100° C, apressure of 1 × 10.sup.⁻⁵ torr, a furnace time of less than 12 hours isrequired to migrate the metal-rich droplet 22 through a silicon body 10of 1 centimeter thickness. The completed structure after processing isshown in FIG. 4.

The thermal migration of the droplet 22 forms a region 24 ofrecrystallized material of the body 10 having solid impurity of themetal 20 therein. The conductivity type of the material of the region 24is a different and opposite type thereby forming a P-N junction by thecontiguous surfaces of the materials of opposite type conductivity. Theresistivity of the region 24 is dependent on the metal migrated throughthe body 10.

It has been discovered that when the body 10 is of silicon, germanium,silicon carbide, gallium arsenide semiconductor material and the like,the droplet 22 has a preferred shape which also gives rise to the region24 being of the same shape as the droplet 22. In a crystal axisdirection of < 111 > of thermal migration, the droplet 22 migrates as atriangular platelet laying in a (111) plane. The platelet is bounded onits edges by (112) planes. A droplet 22 larger than 0.10 centimeter oran edge is unstable and breaks up into several droplets duringmigration. A droplet 22 smaller than 0.0175 centimeter may not migrateinto the body 10 because of a surface barrier problem.

The ratio of the droplet migration rate over the applied thermalgradient is a function of the temperature at which thermal migration ofthe droplet 22 is practiced. At high temperatures, of the order of from1100° C to 1400° C, the droplet migration velocity increases rapidlywith increasing temperature. A velocity of 10 centimeters per day or 1.2× 10.sup.⁻⁴ centimeter per second is obtainable for aluminum droplets insilicon.

The droplet migration rate is also affected by the droplet volume. In analuminum-silicon system, the droplet migration rate decreases by afactor of two when the droplet volume is decreased by a factor of 200.

A droplet 22 migrates in the < 100 > crystal axis direction as apyramidal bounded by four forward (111) planes and a rear (100) plane.Careful control of the thermal gradient and migration rate is anecessity. Otherwise, a twisted region 24 may result. It appears thatthere is a non-uniform dissolution of the four forward (111) facets inthat they do not always dissolve at a uniform rate. Non-uniformdissolution of the four forward (111) facets may cause the regularpyramidal shape of the droplet to become distorted into a trapezoidalshape.

For a more thorough understanding of the temperature gradient zonemelting process and the apparatus employed for the process, one isdirected to our copending applications entitled Method of Making DeepDiode Devices, U.S. Pat. No. 3,901,736; High Velocity Thermal MigrationMethod of Making Deep Diodes, U.S. Pat. No. 3,918,801; Deep DiodeDevices and Method and Apparatus, now abandoned in favor of Ser. No.552,154; High Velocity Thermomigration Method of Making Deep Diodes,U.S. Pat. No. 3,898,106; Deep Diode Device Having Dislocation-Free P-NJunctions and Method, U.S. Pat. No. 3,902,925; and the StabilizedDroplet Method of Making Deep Diodes Having Uniform ElectricalProperties, U.S. Pat. No. 3,899,361; filed concurrently with this patentapplication and assigned to the same assignee of this invention.

The regions of recrystallized material exhibits substantiallytheoretical physical values depending upon the materials involved.Various materials may be migrated into the body 10 to provide variousresistivities and conductivity types therein.

Upon completion of the thermal migration of the metal droplets to formthe columnar array, selective etching and the like is employed to removethe remaining layer 16 of silicon oxide and any damaged material fromthe surface 12. The surface 12 may be processed to remove material toeradicate the depressions 18. Alternatively, the depression 18 may beleft on the surface 12. Referring now to FIG. 5, layers 30 and 32 of anelectrically insulating material such, for example, as silicon oxide,silicon nitride, aluminum oxide and the like are disposed on therespective surfaces 12 and 14 of the processed body 10 by any of themethods well known to those skilled in the art. Employingphotolithographical techniques and selective etching well known to thoseskilled in the art, windows 34 and 36 are opened in the respectivelayers 30 and 32 to expose selective end surface areas of each region24. The exposed portions of the P-N junction 26 in the surfaces 12 and14 are still protected by the respective insulating layers 30 and 32.Again employing photolithographical techniques and selective etching aplurality of electrical contacts 38 comprising a suitable metal such,for example, as tin, aluminum, gold and the like are disposed on therespective layers 30 and 32 and exposed end surfaces of selectives ofthe regions 24 therein and so arranged as to produce a simple or complexsolid state helical coil 40 in the body 10. The coil 40 functions as oneof the windings of a solid state transformer. In a similar manner,electrical insulating layers 46 and 48 are disposed in the respectivelayers 30 and 32 and contacts 38. Photolithographical techniques andselective etching is employed to expose selective end surfaces of theremaining regions 24 and electrical contacts 50 affixed thereto tointerconnect these remaining columnar regions 24 into a serieselectrical circuit which functions as an induction coil 52. The coil 52functions as another winding of a solid state transformer. Electricalleads 54 and 56 are connected to the coil 52 to connect the coil intoexternal electrical circuitry. Windows 58 are opened in the layers 46and 30 by suitable means as described before to enable electrical leads42 and 44 to be affixed to the coil 40 in order to connect the coil 40into the external electrical circuit.

When employed in integrated circuits and the like, it is preferred thata deep diode solid state transformer be electrically isolated from theother electrical devices in the common substrate which they share.Therefore, with reference to FIG. 5 again, and embodying the process ofthermal migration of metal wires in a manner similar to the thermalmigration of the metal droplets 22, an electrically insulating gridcomprising P-type conductivity regions 60 and accompanying P-N junctions62 is formed in the body 10. The grid comprises regions 60 which mayextend the full width and depth of the body 10 or a plurality ofintersecting planar regions 60 may be employed to electrically isolatethe transformer from the remainder of electrical circuits and devices inthe body 10. For a more thorough discussion of electrical isolationgrids and process of making the same, one is directed to the followingcopending applications, which are filed on the same day as this patentapplication and assigned to the same assignee, entitled "IsolationJunctions For Semiconductor Devices," Ser. No. 411,012, now abandoned infavor of application Ser. No. 556,726; and "Thermomigration ofMetal-Rich Liquid Wires Through Seimiconductor Materials," U.S. Pat. No.3,899,362.

Although the solid state transformer is shown as a simple primary andsecondary winding configuration, one may make other configurationseasily by employing the appropriate size array of regions 24 and theproper electrical interconnecting patterns.

The thermal migration of metal wires is preferably practiced inaccordance with the planar orientations, migration directions, stablewire directions and stable wire sizes of the following Table:

                  Table                                                           ______________________________________                                         Wafer  Migration  Stable Wire  Stable Wire                                   Plane   Direction  Directions   Sizes                                         ______________________________________                                        (100)   < 100 >          < 011 >* 100 microns                                                          < 0- 11 >*                                                                             100 microns                                 (110)   < 110 >          < 1- 10 >*                                                                             150 microns                                 (111)   < 111 >    (a)   < 01- 1 >                                                                     < 10- 1 >                                                                              500 microns                                                          < 1- 10 >                                                               (b)   < 11- 2 >*                                                                    < - 211 >*                                                                             500 microns                                                          < 1- 21 >*                                                              (c)   Any other*                                                                    Direction in                                                                           500 microns                                                          (111) plane*                                         ______________________________________                                         *The stability of the migrating wire is sensitive to the alignment of the     thermal gradient with the < 100 >, < 110 > and < 111 > axis, respectively     +Group a is more stable than group b which is more stable than group c.  

The invention has been described relative to practicing thermal gradientzone melting in a negative atmosphere. However, it has been discoveredthat when the body of semiconductor material is a thin wafer of theorder of 10 mil tickness, the thermal gradient zone melting process maybe practiced in the inert gaseous atmosphere of hydrogen, helium, argonand the like in a furnace having a positive atmosphere.

We claim as our invention:
 1. A solid state transformer comprisinga bodyof semiconductor material having two opposed major surfaces formingrespectively the top and bottom surfaces of the body, a predeterminedlevel of resistivity, a predetermined first type conductivity, apreferred crystal structure and a vertical axis substantiallyperpendicular to the major opposed surfaces and a first preferredcrystal axis of the material; at least one of the opposed major surfaceshaving a predetermined crystal planar orientation which is one selectedfrom the group consisting of (100), (110) and (111); a plurality ofregions of recrystallized semiconductor material of the body having asecond and opposite type conductivity than that of the body and apredetermined level of resistivity formed in the body and each of whichhas a vertical axis so oriented as to be aligned approximately parallelwith the first preferred crystal axis; each of the plurality of regionsextending between, and terminating in, the opposed surfaces andproviding a low electrical resistance path for conducting an electricalcurrent between the opposed major surfaces; the recrystallized materialof each region is formed in situ by the migration of a melt ofmetal-rich semiconductor material of the body by thermal gradient zonemelting at a predetermined elevated temperature along a thermal gradientsubstantially parallel with the first crystal axis and the vertical axisof the body and has a predetermined level of concentration of the metalof the melt as determined by the solid solubility limit of that metal inthat semiconductor material at that predetermined elevated temperatureof migration and the metal is distributed substantially uniformlythroughout the entire region; the metal consisting of at least onedopant impurity material to impart the type conductivity and level ofresistivity to the recrystallized material of the region; a P-N junctionby the contiguous surfaces of the recrystallized material of each regionand the material of the body; first means for electrically connectingselective ones of the plurality of regions into a series electricalcircuit arrangement to function as a primary winding of the transformer,and second means for electrically connecting the remaining ones of theplurality of regions into a series electrical circuit arrangement so asto function as a secondary winding of the transformer.
 2. The solidstate transformer of claim 1 whereinthe preferred planar crystalorientation is (110), and the first crystal axis is < 110 >.
 3. Thesolid state transformer of claim 1 whereinthe preferred planar crystalorientation is (110), and the first crystal axis is < 110 >.
 4. Thesolid state transformer of claim 1 whereinthe semiconductor material isone selected from the group consisting of silicon, silicon carbide,germanium and gallium arsenide.
 5. The solid state transformer of claim4 whereinthe semiconductor material is silicon having N-typeconductivity and the solid solubility dopant material is aluminum. 6.The solid state transformer of claim 5 whereinthe concentration of thealuminum is 2 × 10¹⁹ atoms per cubic centimeter.
 7. The solid statetransformer of claim 1 whereineach P-N junction is a step junction. 8.The solid state transformer of claim 1 whereinthe preferred planarcrystal orientation is (111), and the first crystal axis is < 111 >. 9.The solid state transformer of claim 1 whereinthe preferred planarcrystal orientation is (100), and the first crystal axis is < 100 >. 10.The solid state transformer of claim 4 whereinthe preferred planarcrystal orientation is (111), and the first crystal axis is < 111 >. 11.The solid state transformer of claim 5 whereinthe preferred planarcrystal orientation is (111), and the first crystal axis is < 111 >. 12.The solid state transformer of claim 4 whereinthe preferred planarcrystal orientation is (100), and the first crystal axis is < 100 >. 13.The solid state transformer of claim 5 whereinthe preferred planarcrystal orientation is (100), and the first crystal axis is < 100 >.